The present invention relates to a compound semiconductor device such as a HEMT (High Electron Mobility Transistor).
A HEMT structure is attracting attention as a field-effect transistor making use of a two-dimensional electron gas accumulated in a heterojunction interface. As shown in FIG. 1, this HEMT structure comprises an undoped semiconductor layer 2 disposed on a substrate 1; a doped semiconductor layer 3 whose electron affinity is smaller than that of the undoped semiconductor layer 2 and in which impurities are doped; a gate electrode 4 formed on the doped semiconductor layer 3; and a source electrode 6 and a drain electrode 7 which are respectively formed on a cap layer 5 at both sides of the gate electrode 4. Though the doped semiconductor layer 3 located immediately below the gate electrode is not completely depleted in a state in which the gate voltage is not applied, the two-dimensional electron gas accumulated in the heterojunction interface is directly controlled by the gate voltage which is under 0 volt.
However, with the conventional HEMT structure in which the doped semiconductor layer located immediately below the gate electrode is completely depleted in the state in which the gate voltage is not applied, the source resistance is large, and the noise characteristic in a high frequency is deteriorated. To explain this point, first, the source resistance is expressed by the sum of the contact resistance between the source electrode 6 and the cap layer 5 and the sheet resistance of the epitaxial growth layer. As shown in FIG. 1, the sheet resistance of the epitaxial growth layer is further divided into portions (A) and (B), as shown in FIG. 1, and the resistance is particularly large in portion (B). The reason for this is that a plurality of current paths exist in portion (A), as shown at I1 and I2, and its resistance is therefore relatively small, whereas the number of paths is small in portion (B), as shown at I3.
An energy band in a state in which the gate voltage is not applied and the doped semiconductor layer located immediately below the gate electrode is completely depleted, is slightly high as a whole for both portion (B) and the portion located immediately below the gate electrode, as shown by the solid line in FIG. 5. As such, practically no electrons remain in the doped semiconductor layer located immediately below the gate electrode and, likewise, practically no electrons remain in the doped semiconductor layer in portion (B). Then, when a negative voltage is applied to the gate electrode, the energy band for portion (B) is not as high as at the portion located immediately below the gate electrode, but rises as shown by the dotted line in FIG. 5. For this reason, the amount of electrons becomes small at portion (B) where the amount thereof is originally small. Accordingly, in a state of use, the electrons at portion (B) are not sufficient, and a resistance value becomes large, so that the noise is correspondingly large.
To lower the resistance at portion (B), it suffices to increase the amount of electrons by increasing the impurity concentration of the doped semiconductor layer, but if the concentration is made excessively high, the portion of the doped semiconductor layer located immediately below the gate does not become depleted when the gate voltage is applied. Hence, if the device is operated in that state, it is impossible to obtain a satisfactory characteristic.